1. Field of the Invention
The present invention relates to a pressure control butterfly valve used for controlling discharge air of micro flow rate while securing high sealability in a semiconductor fabrication process, for example, and particularly to a pressure control butterfly valve provided with a highly durable isolation function suitable for slowpump control from atmospheric pressure to rough vacuum and for pressure control for stably controlling process gas pressure from a vacuum state at high speed.
2. Description of the Prior Art
Conventionally, a structure has been proposed, in which a pressure control valve with an isolation function is disposed between a vacuum chamber that is a vacuum container and a vacuum pump in a semiconductor fabrication process, for example. The vacuum isolation valve (hereinafter referred to as the “isolation valve”) serves to perform both discharge air from the vacuum chamber and stop of the discharge air and to control the internal pressure of the vacuum chamber to approximate prescribed vacuum pressure. In the case of performing pressure control using the isolation valve, it is required to enable performing two controls that are slowpump control and process gas control.
In the case of controlling pressure using the isolation valve, when a valve travel has become abruptly large in vacuum discharge air from atmospheric pressure, the pressure in the vacuum chamber is rapidly changed to generate turbulence, resulting in particles possibly flying in all directions. In order to prevent this, slowpump has heretofore been performed using a small-bore bypass valve disposed in parallel to a main valve. According to the pressure control valve with an isolation function, however, it has become possible to leak a fluid while varying a flow passage in valve travel in dribs and drabs in the vicinity of a place at which a large-bore valve is closed and, in this case, pressure control becomes necessitated in a state of a minute valve travel.
The isolation valve includes a pendulum gate valve, a direct-acting gate valve, an L-shaped valve and a butterfly valve, for example. Since the pendulum gate valve or direct-acting gate valve has a structure making a gap at a valve-closed position large, it is used in many cases for pressure control in a large-bore high-vacuum discharge air system.
As the L-shaped valve, a vacuum proportional control valve of a vacuum pressure control system disclosed in JP-A 2006-18767 can be cited. The vacuum pressure control system of the prior art reference is equipped with the vacuum proportional control valve that is an L-shaped poppet valve, a vacuum pressure sensor and a controller. The vacuum proportional control valve has a valve seat-sealing function, a slowpump function and a pressure control function and, with these functions, serves to solely perform vacuum pressure control.
On the other hand, as a butterfly valve with an isolation function, there is a butterfly valve enabling isolation through opening/closing of a seat ring (refer, for example, Japanese Utility Model Registration No. 2516307). The butterfly valve of the prior art reference has a structure which is provided in a valve disc rotation spacing of the inside of a valve body with a valve disc save space and in which a valve disc is rotated while preventing contact with the inner surface of the valve disc rotation spacing during the rotation of the valve disc. A valve seat thereof advances in the valve disc rotation spacing by the pressure supplied into a pressure space to come into pressure contact with the valve disc, thereby sealing a flow passage. As a result, the butterfly valve prevents contact of the valve disc with the valve body and valve seat during the rotation of the valve disc to prevent wear or damage of the valve disc and enhance the sealability.
Generally, in this case, the butterfly valve has a small face-to-face dimension as compared with a same-bore different-structure valve and a small width for attachment to a pipe as compared with a poppet flow control valve. For this reason, when the butterfly valve is used as an isolation valve, a footprint property is enhanced and, when it is used particularly in the semiconductor fabrication field, it is possible to downsize an overall exhaust system. Furthermore, the butterfly valve can be made lightweight to enable reduction of the material for the same.
Japanese Patent No. 3826114 discloses another butterfly valve having a valve disc opening/closing mechanism. In the valve disc opening/closing mechanism of the butterfly valve, a valve seat seal part is slidable between a seal position at which the seal part comes into intimate contact with the outer circumferential surface of a valve disc and a save position at which the seal part departs from the outer circumferential surface of the valve disc, and a contraption of jetting a cleansing fluid into a gap between the valve seat seal part and the valve body is adopted. Therefore, the butterfly valve intends to remove solid substances to be urged to enter between the valve seat seal part and the valve body, thereby preventing wear or damage of the valve seat seal part or valve body to enhance the sealability.
In the meantime, since a butterfly valve has a structure in which a valve shaft is rotated relative to a valve seat to open or close a flow passage, in the case where a flapper (valve disc) is provided with no sealing material and has no isolation function, it can be operated at high speed to enable flow control. Here, in a butterfly valve provided with a sealing material and having an isolation function, for example, the sealing material will greatly be worn away by rotation and, even when it is coated with a vacuum grease, the sealing area will be worn away by slide, thus lessening durability. This butterfly valve reaches its application limits after several thousand rotations, and a heating-type butterfly valve is further inferior in durability. However, a butterfly valve having no isolation function will suffice if a section to be rotated has high sealability and, since a sealing area is not required to have durability, the butterfly valve can endure even after around a million of rotations.
In the case of controlling vacuum pressure using a butterfly valve having no isolation function, there is a case where a valve system having a combination of this butterfly valve and a different valve is used to control vacuum system. An example of the valve system is shown in FIG. 11. The valve system 1 comprises a pressure control butterfly valve 2 having no isolation function, a vacuum valve 3 for opening/closing a flow passage and a small-bore flow control valve 4. The butterfly valve 2 and vacuum valve 3 are connected in series relative to a vacuum flow passage 5. The flow control valve 4 is connected to the vacuum valve 3 in parallel to the vacuum flow passage 3 to enable bypassing the vacuum valve 3. The flow control valve 4 is used to adjust the flow rate to adjust the discharge air time of slowpump.
The valve system 1 has a configuration in which the vacuum valve 3 is used to open and close the flow passage and the butterfly valve 2 is controlled to be opened or closed at the time of closing the vacuum valve 3, thereby controlling pressure. In addition, in the case of slowpump, the small-bore flow control valve 4 is opened, with the vacuum valve 3 brought to a closed state, to perform slowpump from atmospheric pressure to rough vacuum. With this configuration, the valve system 1 can adjust the discharge air time of slowpump difficult to perform with a butterfly valve alone.
However, the pendulum and direct-acting gate valves have a structure suitable for pressure control of a large-bore high-vacuum discharge air system and have a complicated body structure as compared with the body structure of a butterfly valve. Therefore, the structure of the gate valves is suitable not for fulfilling a slowpump function with an integral model, but for controlling pressure with an intermediate or small model.
Furthermore, since the L-shaped poppet valve is movable so that the valve disc may reciprocate in the direction of a flow passage, the size thereof become large in order to secure a wide movable region of the valve disc and a wide flow passage assumed when the valve disc ascends. For this reason, the poppet valve exhibits a bad footprint property and has a tendency to enlarge the size of a discharge air system as a whole. Furthermore, since the poppet valve adopts a system of controlling the opening/closing motion of a large-bore air cylinder with an electropneumatic positioner, it cannot attain high-speed control.
In the vacuum pressure control system of JP-A 2006-18767, for example, the valve opening-closing stroke of the vacuum proportional control valve becomes large and, moreover, since a bellows is used as a valve seal, high thrust is required for the purpose of operating the control valve. Thus, it is necessary for a pneumatic cylinder to have high thrust in order to operate the vacuum proportional control valve. In addition, since the vacuum pressure control system has the structure of a valve-seat in which the vacuum proportional control valve is sealed with the flat surface, when it has been used for micro control of the slowpump, the pneumatic control is bad in controllability and a problem of hunting etc. due to adhesion of the valve seat will be entailed. For this reason, there is a case where control by mechanical drive of a motor has been adopted and, in this case, however, the system becomes large-sized and the structure thereof becomes complicated.
On the other hand, in the case of using the butterfly valve for vacuum control, in order to cause the butterfly valve to serve as an isolation valve, it is necessary that the valve disc be provided with a sealing material and, in order to enhance the valve seat sealability, that the pressure for fastening the valve disc and the valve seal part be heightened to enhance the adhesion between them. In this case, however, frictional torque exerted when the valve seat has been fastened will become large.
For this reason, in the case of performing pressure control using the butterfly valve having a high valve seat-sealing function, it is necessary that the output of a motor and the reduction ratio of gears be set to be large to increase output torque. However, the increase in output torque delays the operation speed to disable a high-speed control function. In addition, when the size of the motor is made large and the number of the gears is increased, the size of the valve as a whole becomes large. Furthermore, the valve disc and valve seat are easy to wear away to impair the leak performance. Therefore, in the butterfly valve having the isolation function, the limit of the number of rotations of the valve disc will be around hundred thousand times and, moreover, in the case of the heating-type butterfly valve, the limit of the number of rotations will be reduced to lessen the durability thereof. These are problematic.
In view of the above, in the butterfly valve of Japanese Utility Model Registration No. 2516307, it is tried to enhance the sealability through providing the valve disc rotation spacing with the valve disc save space to prevent the valve disc from being worn away. However, since the valve body is formed largely in the diameter expansion direction to form the valve disc save space, the size of the body as a whole becomes large and the width for attachment is increased. Therefore, the butterfly valve of the prior art reference fails to maintain its miniaturization that is one of the merits ordinary butterfly valves have.
Furthermore, in the valve disc opening/closing mechanism of the butterfly valve of Japanese Patent No. 3826114, it is tried to isolate the valve seat seal part from the valve disc. In order to prevent interference between the valve disc and the valve seat seal part during the rotation of the valve disc, however, it is necessary for the valve seat seal part to be isolated greatly from the valve disc and, as a result, the conductance during a small flow rate becomes large. Therefore, it is difficult for the conventional butterfly valve to control the pressure during the small flow rate and unsuitable for vacuum pressure control.
On the other hand, the butterfly valve which has no isolation function and whose valve disc is provided with no sealing material aims mainly at preventing the body and valve seat seal part from being worn away. When the butterfly valve is equipped with a flow control valve with an isolation function, which is disposed in parallel to a poppet valve attached for on-off control to a main pipe of 100 A and when slowpump is performed with a flow control valve having a size of ¼ inch, for example, a gap of around 0.1 mm between a valve disc and a seal part is required in order to control an area of ¼ inch with a butterfly valve having a size of 100 A. However, the butterfly valve having an opening/closing function alone has a gap made larger than necessary from a design standpoint and exhibits inferior min controllability. Therefore, it is impossible to adjust the time of slowpump with the butterfly valve having the opening/closing function alone.
In order to perform slowpump utilizing a large-bore butterfly valve, such as a butterfly valve of 100 A, it is necessary to control opening and closing at separating and contacting positions. In the case of making it impossible to reduce a gap between the valve disc and the valve seat to 0.1 mm or less, the conductance during micro flow rate becomes large as described above. In this case, the range of pressure controllable will be narrow to make it impossible to control the pressure in a sufficient range. However, when the sealability in the vicinity of only the section to be rotated is made high, like this valve, since the part at which wear and leak are mainly induced is in the vicinity of the seal part, there is a merit of enabling high-speed control owing to enhanced durability and small wear resistance.
The valve system 1 shown in FIG. 11 can adjust the discharge air time of the slowpump and control the pressure of the process gas, while it entails the problems of a complicated structure and a large space for attachment to the vacuum pipe 5.
The present invention has been developed as a result of keen studies made in view of the aforementioned problems. The object thereof is to provide a pressure control butterfly valve suitable for controlling the internal pressure of a vacuum region, having excellent durability, solely fulfilling an isolation function, capable of performing both control of the discharge air time of slowpump from atmospheric pressure to rough vacuum and control of the pressure of process gas, only requiring a small space for installation, maintaining a valve-closed state during closing the valve to attain high sealability, and capable of accurately controlling flow rates from a micro flow rate to a large flow rate during the flow control.